The development of technologies to determine the position of a mobile device has enabled application developers and wireless network operators to provide location-based and location-aware services. Examples of these are guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services that give the mobile user information about his or her surroundings or that use this information to enhance their services.
In addition to the commercial services facilitated by these technologies, location-based emergency services are also being deployed. The governments in several countries have put specific requirements on the network operators to be able to determine the position of an emergency call. For instance, governmental requirements in the United States specify that mobile networks must be able to determine the position of a certain percentage of all emergency calls and further include accuracy requirements. The requirements make no distinctions between indoor and outdoor environments.
In many environments, the position can be accurately estimated by using positioning methods based on Global Navigation Satellite Systems (GNSS), such as the well-known Global Positioning System (GPS). However, GPS-based positioning may often have unsatisfactory performance, especially in urban and/or indoor environments.
Complementary positioning methods may also be provided by a wireless network to augment GPS technology. In addition to mobile terminal-based GNSS (including GPS), the following methods are currently available or will be soon be included in the Long-Term Evolution (LTE) standards developed by the 3rd-Generation Partnership Project (3GPP):                Cell ID (CID),        E-CID, including network-based angle-of-arrival (AoA),        Assisted-GNSS (A-GNSS), including Assisted-GPS (A-GPS), based on satellite signals,        Observed Time Difference of Arrival (OTDOA),        Uplink Time Difference of Arrival (UTDOA)—currently being standardized.        
Several positioning techniques are based on time-difference-of-arrival (TDOA) or time-of-arrival (TOA) measurements. Examples include OTDOA, UTDOA, GNSS, and Assisted-GNSS (A-GNSS). A typical, though not the only, format for the positioning result with these techniques is an ellipsoid point with an uncertainty circle/ellipse/ellipsoid, which is the result of intersection of multiple hyperbolas/hyperbolic arcs (e.g., OTDOA or UTDOA) or circles/arcs (e.g., UTDOA, GNSS, or A-GNSS).
Several techniques, such as Adaptive Enhanced Cell Identity (AECID), may involve a mix of any of the methods above, and are thus regarded as “hybrid” positioning methods. With these methods, the position result can be almost any shape, but in many cases it is likely to be a polygon.
Cellular-based positioning methods (as opposed to satellite-based methods, for example) rely on knowledge of anchor nodes' locations, i.e., the fixed locations from which measured signals are transmitted (e.g., for OTDOA) or the fixed locations at which signals transmitted by mobile devices are measured (e.g., for UTDOA). These fixed locations may correspond, for example, to base station or beacon device locations for OTDOA, Location Measurement Unit (LMU) antenna locations for UTDOA, and base station locations for E-CID. The anchor nodes' locations may also be used to enhance AECID, hybrid positioning, etc.
Positioning Architecture
In 3GPP, location-based services are known as Location Services (LCS). Three key network elements in an LTE positioning architecture are the LCS Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity that manages positioning for a LCS target device by collecting measurements and other location information, assists the target device in measurements when necessary, and estimating the LCS target location. A LCS Client is a software-based and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e., the entities being positioned. LCS Clients may reside in a network node, an external node (i.e., a network external to a cellular network), a Public Safety Access Point (PSAP), a user equipment (or “UE,” in 3GPP terminology for an end-user wireless station), a radio base station (or “eNodeB,” in LTE systems), etc. In some cases, the LCS Client may reside in the LCS target itself. An LCS Client (e.g., an external LCS Client) sends a request to LCS Server (e.g., a positioning node) to obtain location information. The LCS Server processes and services the received requests and sends the positioning result (sometimes including a velocity estimate) to the LCS Client.
In some cases, the position calculation is conducted by a positioning server, such as an Enhanced Serving Mobile Location Center (E-SMLC) or a Secure User-Plane Location (SUPL) Location Platform (SLP) in LTE. In other cases, the position calculation is carried out by the UE. The latter approach is known as the UE-based positioning mode, while the former approach includes both network-based positioning, i.e., position calculation in a network node based on measurements collected from network nodes such as LMUs or eNodeBs, and UE-assisted positioning, where the position calculation in the positioning network node is based on measurements received from UE.
LTE Positioning Protocol (LPP) is a positioning protocol for control plane signaling between a UE and an E-SMLC, which is used by the E-SMLC to provide assistance data to the UE and by the UE for reporting measurements to the E-SMLC. LPP has been designed in such a way that it can also be utilized outside the control plane domain such as in the user plane in the context of SUPL. LPP is currently used for downlink positioning.
LTE Positioning Protocol Annex (LPPa), sometimes referred to as LTE Positioning Protocol A, is a protocol between the eNodeB and the E-SMLC, and is specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information. For example, LPPa can be used to retrieve information such as positioning reference symbol (PRS) configuration in a cell for OTDOA positioning, or UE sounding reference signal (SRS) configuration for UTDOA positioning, and/or eNodeB measurements. LPPa may be used for downlink positioning and uplink positioning.
FIG. 1 illustrates the UTDOA architecture currently under discussion in 3GPP, including nodes found in the Radio Access Network (RAN) and the core network, as well as an external LCS Client. Although uplink (UL) measurements may in principle be performed by any radio network node, such as the illustrated LTE eNodeB 110, the UL positioning architecture also includes specific UL measurement units, known as Location Measurement Units (LMUs), which are logical and/or physical nodes that measure signals transmitted by a target UE, such as the UE 130 illustrated in FIG. 1. Several LMU deployment options are possible. For example, referring to FIG. 1, LMU 120a is integrated into eNodeB 110, while LMU 120b shares some equipment, e.g., at least antennas, with eNodeB 110. LMU 120c, on the other hand, is a standalone physical node comprising its own radio components and antenna(s).
While the UTDOA architecture is not finalized, there will likely be communication protocols established for communications between a LMU and positioning node, and there may be some enhancements to support UL positioning added to the existing LPPa or to similar protocols.
In LTE, UTDOA measurements, known as UL relative time-of-arrival (RTOA) measurements, are performed on Sounding Reference Signals (SRS). To detect an SRS signal, an LMU 120 needs a number of SRS parameters to generate an SRS sequence that is correlated against the received signal. These parameters are not necessarily known to LMU 120. Thus, to allow the LMU to generate the SRS sequence and detect the SRS signals transmitted by a UE, SRS parameters must be provided in the assistance data transmitted by the positioning node to LMU; these assistance data would be provided via SLmAP. However, these parameters may generally be not known to the positioning node, which needs then to obtain this information from eNodeB configuring the SRS to be transmitted by the UE and measured by LMU; this information (e.g., SRS transmit configuration or the updated SRS configuration) would have to be provided in LPPa.
Example parameters that may be signaled over LPPa from eNodeB to E-SMLC for UL/UTDOA positioning may comprise, e.g., those parameters illustrated in Table 1, below. Note that many of these parameters are described in the latest version of 3GPP document 3GPP TS 36.211, available at www.3gpp.org.
TABLE 1CategoryParametersGeneralPCI of PCellUL-EARFCN of PCellTiming advance measurement for the UE in PCellSRSFor each serving cell in which SRS is configured:PCIUL-EARFCNUL cyclic prefixUL system bandwidth of the cellCell-specific SRS bandwidth configurationsrs-BandwidthConfigUE-specific SRS bandwidth configuration srs-Bandwidthnumber of antenna ports for SRS transmissionsrs-AntennaPortfrequency domain positionSRS frequency hopping bandwidth configurationSRS-Cyclic shiftTransmission combSRS configuration indexMaxUpPt, used for TDD onlyGroup-hopping-enableddeltaSS, parameter Δss, included when SRS sequencehopping is used and not included otherwise
Example parameters that may be signaled over SLmAP from E-SMLC to LMU(s) may comprise, e.g., those shown in Table 2, below. Again, many of these parameters are described in the latest version of 3GPP document 3GPP TS 36.211, available at www.3gpp.org.
TABLE 2CategoryParametersGeneralSearch window parameters:expected propagation delay, T, corresponding to distancebetween LMU and PCell, delay uncertainty ΔSRSFor each serving cell in which SRS is configured andto be measured by LMU:PCIUL-EARFCNUL cyclic prefixUL system bandwidth of the cellCell-specific SRS bandwidth configurationsrs-BandwidthConfigUE-specific SRS bandwidth configuration srs-Bandwidthnumber of antenna ports for SRS transmissionsrs-AntennaPortfrequency domain positionSRS frequency hopping bandwidth configurationSRS-Cyclic shiftTransmission combSRS configuration indexMaxUpPt, used for TDD onlyGroup-hopping-enableddeltaSS, parameter Δss, included when SRS sequencehopping is used and not included otherwise
Measurements for UL positioning and UTDOA are performed on UL transmissions, which may include, for example, reference signal transmissions or data channel transmissions. UL RTOA is the currently standardized UTDOA timing measurement, and may be performed on Sounding Reference Signals (SRS). The results of the measurements are signaled by the measuring node (e.g., LMU) to the positioning node (e.g., E-SMLC), e.g., over SLmAP.
FIG. 2 illustrates the current architecture under discussion in 3GPP for downlink (DL) positioning, again including nodes found in the Radio Access Network (RAN) and the core network, as well as an external LCS Client. It will be appreciated that this architecture includes many of the same components found in the UL positioning architecture illustrated in FIG. 1. Two additional components shown in FIG. 2, however, are the Serving Gateway (S-GW) and the Packet Data Network Gateway (PDN GW, or P-GW). These gateways terminate the UE's interfaces towards the E-UTRAN network and the Packet Data Network (PDN), respectively. LPP is currently used for downlink positioning. An LPP message may also include an LPP extension packet data unit (EPDU); Open Mobile Alliance (OMA) LPP Extensions, defined as LPPe, take advantage of this possibility. Currently, LPP and LPPe are used mainly for downlink positioning, while LPPa may be used both for DL and UL positioning.
Positioning Result
A positioning result is a result of processing of obtained measurements, including Cell IDs, power levels, received radio signal strengths or quality, etc. The positioning result is often based on radio measurements (e.g., timing measurements such as timing advance and RTT, or power-based measurements such as received signal strength, or direction measurements such as angle-of-arrival measurements) received from measuring radio nodes (e.g., UE or eNodeB or LMU).
The positioning result may be exchanged among nodes in one of several pre-defined formats. The signaled positioning result is represented in a pre-defined format, e.g., corresponding to one of the seven Universal Geographical Area Description (GAD) shapes.
Currently, a positioning result may be signaled between:                an LCS target, e.g., a UE, and an LCS server, e.g., over LPP protocol;        two positioning nodes, e.g., an E-SMLC or SLP, e.g., over a proprietary interface;        a positioning server (such as an E-SMLC,) and other network nodes, e.g., a Mobility Management Entity (MME), a Mobile Switching Center (MSC), a Gateway Mobile Location Center (GMLC), an Operations and Maintenance (O&M) node, a Self-Organizing Network (SON) node, and/or a Minimization of Drive Tests (MDT) node;        a positioning node and an LCS Client, e.g., between an E-SMLC and a Public Safety Access Point (PSAP), or between an SLP and an External LCS Client, or between an E-SMLC and a UE.Note that in emergency positioning, the LCS Client may reside in a PSAP.        
The result for UL positioning is based at least on one UL measurements. UL measurements may also be used for hybrid positioning. UL measurements may be used jointly with other measurements, to obtain the positioning result.
Uplink Positioning Measurements
As the name suggests, measurements for uplink positioning (e.g., UTDOA) are performed on uplink transmissions, which may comprise, e.g., one or more of physical signal or channel transmissions, e.g., reference signal transmissions, random access channel transmissions, Physical Uplink Control Channel (PUCCH) transmissions, or data channel transmissions. Some examples of reference signals transmitted in LTE UL are SRS and demodulation reference signals.
UL Relative Time of Arrival (RTOA) is a currently standardized UTDOA timing measurement. The measurement may be performed on Sounding Reference Signals (SRS), which may be configured for periodic transmissions, typically comprising multiple transmissions but may also be one transmission. SRS transmissions may be triggered by any of the two trigger types:                Trigger type 0: higher layer signaling from eNodeB,        Trigger type 1: via downlink control channel signaling (DCI formats 0/4/1A for FDD and TDD and DCI formats 2B/2C for TDD).        
Other example uplink measurements are the uplink measurements specified in 3GPP TS 36.214. These measurements include measurements of received signal strength, received signal quality, angle-of-arrival (AoA), eNodeB receive-to-transmit (Rx-Tx) timing, relative time-of-arrival (RTOA), and other measurements performed by radio network nodes (e.g., eNodeB or LMU). Other known measurements are UL TDOA, UL TOA, UL propagation delay, etc.
Multi-Antenna Systems
A multi-antenna system may use one or more multi-antenna transmit and/or multi-antenna receive techniques, such as Single-User Multiple-Input Multiple-Output (SU-MIMO) or multi-user MIMO (MU-MIMO) techniques, transmit diversity, receive diversity, beam-forming, Antenna Array Systems as currently being standardized in 3GPP, multi-point communication (e.g., coordinated-multipoint, or CoMP), distributed antenna systems (DAS), etc. The antennas associated with a node may be, e.g., co-located, quasi-collocated (e.g., based on some channel properties such delay spread, etc.), or non-collocated.
A multi-antenna system may also deploy Remote Radio Units (RRUs) or Remote Radio Heads (RRHs). An RRU is a single unit in which only the RF front-end functionalities are implemented and which is connected to the remaining baseband processing part (Base Band Unit, or BBU) through a link (e.g., a fiber or wireless link). Depending on the functionality split, an RRU may also comprise some baseband functionality. RRU may also be referred to as a RRH.
A multi-antenna system may also comprise a multi-carrier system operating on multiple carrier frequencies or component carriers (CCs) and/or in different RF bands. The multi-carrier system may use carrier aggregation (CA), as described below, where different CCs may or may not be co-located.
Multi-Carrier or Carrier Aggregation Concept
To enhance peak rates within a technology, so-called multi-carrier or carrier aggregation solutions are known. Each carrier in multi-carrier or carrier aggregation system is generally termed as a component carrier, or sometimes referred to as a cell. In simple terms, the component carrier is an individual carrier in a multi-carrier system. The term carrier aggregation is also referred to with the terms (e.g., interchangeably called) “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. Carrier aggregation is used for transmission of signaling and data in the uplink and downlink directions. One of the component carriers is the primary component carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called secondary component carriers (SCCs) or simply secondary carriers or even supplementary carriers. Generally the primary or anchor component carrier carries the essential UE specific signaling. The primary component carrier exists in both uplink and downlink direction in carrier aggregation. The network may assign different primary carriers to different UEs operating in the same sector or cell.
With carrier aggregation, the UE has more than one serving cell in downlink and/or in the uplink: one primary serving cell and one or more secondary serving cells operating on the PCC and SCCs respectively. The serving cell is interchangeably called the primary cell (PCell) or primary serving cell (PSC). Similarly, the secondary serving cell is interchangeably called the secondary cell (SCell) or secondary serving cell (SSC). Regardless of the terminology, the PCell and SCell(s) enable the UE to receive and/or transmit data. More specifically the PCell and SCell exist in downlink and uplink for the reception and transmission of data by the UE. The remaining non-serving cells are called neighbor cells.
Component carriers belonging to the CA may belong to the same frequency band (intra-band carrier aggregation) or to different frequency bands (inter-band carrier aggregation) or any combination thereof (e.g., two component carriers in band A and one component carrier in band B). Furthermore, the component carriers in intra-band carrier aggregation may be adjacent or non-adjacent in the frequency domain (intra-band, non-adjacent carrier aggregation). A hybrid carrier aggregation comprising any two of intra-band adjacent, intra-band non-adjacent and inter-band aggregations is also possible. Using carrier aggregation between carriers of different technologies is also referred to as “multi-RAT carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of carriers from LTE Frequency-Division Duplex (FDD) and LTE Time-Division Duplexing (TDD) modes, which may also be interchangeably called as multi-duplex carrier aggregation system. Yet another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity, carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The component carriers in carrier aggregation may or may not be co-located in the same site or radio network node (e.g., a radio base station, relay, mobile relay, etc.). For instance, the component carriers may originate at different locations (e.g., from non-co-located base stations, or from base stations and a remote radio head (RRH), or at remote radio units (RRUs)). Well-known examples of combined carrier aggregation and multi-point communication techniques include the Distributed Antenna System (DAS), the Remote Radio Head (RRH), the Remote Radio Unit (RRU), and Coordinated Multipoint (CoMP) transmission. The techniques described herein also apply to multi-point carrier aggregation systems as well as to multi-point systems without carrier aggregation. The multi-carrier operation may also be used in conjunction with multi-antenna transmission. For example signals on each component carrier may be transmitted by the eNodeB to the UE over two or more antennas.
The wide variety of deployments and multi-antenna schemes complicate the selection of and configuration of positioning measurements. Accordingly, improved techniques for selecting and configuring measurement nodes for positioning are needed.